Ionization of Bases in Water: Structure and Stability of the NH4+···OH- Ionic Forms in Ammonia−Water Clusters

The structure and stability of the NH 4 + ···OH - ionic forms resultant from the equilibrium NH 3 + H 2 O ⇌ NH 4 + + OH - is analyzed by Hartree−Fock, second-order Moller−Plesset, and the Becke−Lee−Yang−Parr density functional. Our results show that a double ionic cluster NH 4 + ···(H 2 O) 3 ···OH -, in which three molecules are located between the NH 4 + and OH - ions, is stable and lies 5−10 kcal/mol higher than various neutral clusters containing one ammonia and four water molecules. The process of formation of the double ionic cluster from a geometrically similar neutral cluster is shown to be synchronic, with the two protons being simultaneously transferred. The MP2 and BLYP methods give a similar description of the structure and stability of these clusters.